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#31
You could swap your Windows 10 drive with your Windows XP drive and then switch on the computer without having to change the boot sequence in BIOS, if you need to use XP. After that, you shutdown, swap to your Windows 10 disk and keep using your PC as normal. The only inconvenience is that you have to turn off the computer to change disks while in the standard dual-boot scenario all you have to do is configure Windows 10 boot loader to also boot Windows XP or leave booting from Windows 10 as default and bypass boot sequence (usually F8 or F9 or F11 depending on motherboard model) if you need to boot from XP instead.
PS: Another convenience of having both disks connected is that you can easily share files between the two operating systems.